The present invention relates to a method and apparatus for controlling and adjusting the relative positions of electrode tips of an underwater spark gap device for medical applications, and more particularly to a situation wherein at least one tip of the underwater spark gap device is mounted by at least one electrode holder to a base plate.
The above mentioned underwater spark gap devices are, for example, incorporated in extracorporeal lithotripters, i.e. an instrument for crushing a vesical calculus. By means of a high voltage discharge between the electrode tips, a fluid is spontaneously evaporated, forming a spherically shaped shock front. This shock front can then be focused by an ellipsoidal reflector at a focal point.
Usually one electrode tip of the underwater spark gap device is mounted to a base plate, while the opposite electrode tip is held by an electrode holder comprising a series of rods, forming a kind of cage, which is itself mounted on the above-mentioned base plate.
In technical realizations of these extracorporeal lithotripters, the underwater spark gap devices may include the capability of removable electrode tips (as shown in devices provided by Technomed) or have the capability of interchangeable wearing parts (as shown in devices provided by Dornier). In both cases, intensity, pressure rise time and the reproducibility of the shock wave front depend on the condition and geometry of the electrode tips. The relevant geometric parameters are the distance, the alignment and the angularity of the electrode tips of the electrodes.
The electrode tips have to be adjusted within very narrow tolerances, in order to gain a reproducable spark discharge. This requires an elevated effort during production and a resulting increase in cost. Furthermore, the electrode holder cage may be deformed by mechanical stress, caused by the high pressures of several thousand bar experienced during spark discharge. Another problem is the increase of distance between the electrode tips caused by burndown as a result of normal wear. All the factors lead to a maladjustment of the electrodes which may subsequently lead to malfunction.
These above-noted problems may cause severe risks in that lithotripters based on spark gaps are frequently used in the medical area. The decreased efficacy of improperly adjusted electrode tips results in the requirement of a higher number of shocks during treatment, and subsequently to a prolonging of the time required for treatment and anesthesia of the patient. In the worse case, the therapeutic point of focus of the spark gap device may be dislocated, which means that the zone of maximal pressure is dislocated from the desired treatment area into nearby organ regions. The risk of organ bleedings, which is usually observed following extracorporeal lithotripty, is thus increased, causing stress and pain to the patient. This improper adjustment of the electrode tips may cause peculiar risks, if the treatment areas are located close to vessels, as in the case of treatment of the ureteral area. Lesions of vessels may occur and result in acute risks to the patients' health.
Medical lithogripters currently in use do not allow for the control or adjustment of the geometry of the spark gap. Furthermore, the operator cannot readjust the increased distance between the electrode tips caused by burndown. In order to lower the risks which result from the above-mentioned deficiencies, there is no alternative but to change the complete underwater spark gap assembly, even when this assembly has not been completely worn out, thus causing additional costs per treatment.
Therefore, it is an object of the present invention to provide a method for controlling and readjusting the relative positions of electrodes, forming a so-called underwater spark gap especially for medical applications, at least one electrode being mounted by an electrode holder to a base plate.
A further object of the present invention is to provide an apparatus for carrying out the method for performing the control and readjustment of the geometric parameters of underwater spark gaps.
Thus, certain advantageous embodiments of the present invention readjust the relative position of the electrode tips by a grooved adjustment tool which adapts to the configuration of the electrode holder to position the tips of the electrodes at a nominal position for proper alignment, angularity and electrode distance. By means of a relative movement within the adjustment tool, rods of the electrode holder cage are plastically deformed in such a way that the electrode tip carried by the electrode holder cage is positioned at its nominal position after this procedure.
One particular advantage of the present invention is that if the geometry of the adjustment tool and the freedom of the relative movement within the tool are suitably chosen, the nominal position of the electrode tips can be readjusted by a user without mechanical skills.
The method described with respect to preferred embodiments of the invention also provides the further advantage that it can also be used during the manufacture of underwater spark gaps to simplify the adjustment of the electrode tips during the production procedure and thereby reduce associated production costs.
According to one particular embodiment of the present invention, a mechanical adjustment of the relative position of the electrode tips is performed by an axially symmetrical bending of one or more rods forming the electrode holder cage to thereby achieve the desired adjustment of the alignment, angularity and electrode distance of the electrode tips in a single step.
According to another embodiment of the present invention, the electrode distance can be readjusted independently from alignment and angularity adjustments. This can, for example, be achieved by changing the relative distance between the movable upper and lower segments of the adjustment tool.
Thus, in accordance with certain advantageous embodiments of the apparatus of the present invention, the apparatus includes a dividable adjustment tool for embracing the electrode holder cage of the spark gap device, the adjustment tool being shaped according to the outer contour of the electrode holder cage to position the electrode holder cage to a nominal position at which the tips of the electrodes are properly positioned with respect to alignment, angularity and electrode distance. To perform the adjustment, the adjustment tool for the electrode holder cage is rotated around an electrode axis relative to the holder at the base plate.
In accordance with a further embodiment of the present invention, the dividable adjustment tool comprises a pair of holders, one of the holders being fixed, while the opposite holder is rotated. The fixed holder attaches to the base plate of the spark gap device, while the electrode held by the electrode holder cage is inserted at the rotatable adapter. This arrangement permits a bending of the electrode holder cage to provide a precise readjustment of geometric relationship of the electrode tips.
Preferably, the adjustment tool is rotated around an electrode axis, as this results in a symmetrical bending of all the rods forming the electrode holder cage.
According to additional advantageous embodiments of the present invention, the rods of the electrode cage are only bent in a certain region, namely a region within the adjustment tool in which the rods are not inserted or embraced by the adjustment tool.
The adjustment tool provides the further advantage that it is not only suited to adjust the electrode tips, but also to control their relative positions. Thus, the adjustment tool provides the further feature that it may also be used as gauge to control the position of the electrode tips.
This control can be performed by adding an optical system according to one embodiment of the present invention which allows for the control of the position of the electrode tips visually or by means of an image processing system. Especially by using an image processing system, the method of controlling and adjusting the position of the electrode tips may be performed with an automated system. For example, a servo driven motor may be used to control the movements of the pair of movable holders of the adjustment tool relative to each other.
The control of the position of the electrode tips can also be performed by adding electrical contacts to the adjustment tool embracing the electrode tips, which close an electric circuit to generate a signal at the moment that the electrode tips are in contact with each other. This signal can be processed by an electrical control unit, for example a microprocessor controlled unit, which controls the relative movement of the movable holders of the adjustment tool according to the generated electrical signal mentioned above.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.